T-cell activation involves a two-step process: an antigen-specific signal generated by the TcR.zeta./CD3 complex, followed by a second signal delivered by an accessory cell (June et al., 1990, Immunol. Today 11:211). The TcR.zeta./CD3 complex defines the specificity of recognition, while the co-stimulatory signal is thought to regulate lymphokine expression and proliferation (Mueller et al., 1989, Ann. Rev. Immunol. 7:445; Kohno et al., 1990, Cell. Immunol. 131:1). Engagement of the antigen-receptor in the absence of the co-stimulatory receptor results in clonal non-responsiveness or anergy.
CD28, a disulfide-linked homodimer of 44 kDa expressed on the surface of thymocytes and the majority of T cells (Hara et al., 1985, J. Exp. Med. 161:1513; Moretta et al., 1985, J. Exp Med. 162: 823), is an essential second signal in T cell activation. CD28 is expressed on CD4+ and CD8+ T cells and also on CD4+CD8+ thymocytes (Martin et al., 1986, J. Immunol. 136:3282). Structurally, CD28 is comprised of a single immunoglobulin-like domain and a 51 amino acid cytoplasmic tail (Aruffo and Seed, 1987, Proc. Natl. Acad. Sci. USA 84:8573, herein incorporated by reference). Activation of CD28+ T cells by suboptimal levels of antigen together with anti-TcR.zeta./CD3 and anti-CD2 is augmented by anti-CD28 as measured by proliferation and lymphokine production (June et al., 1987, Mol. Cell Bio 7:4472; Martin et al., 1986, J. Immunol. 136: 3282; Yang et al., 1988, J. Exp. Med. 168:1457; van Lier et al., 1988, Eur. J. Immunol. 18:1753). Binding of antibody to CD28 in the presence of phorbol ester induces mitogenesis (Hara et al., supra), and CD28 signalling initially stabilizes mRNA for various lymphokines, followed by an increase in transcription (Lindsten et al., 1989, Science 244:339).
The natural ligand for CD28 has been identified as B7/BB1 (Linsley et al., 1990, Proc. Natl. Acad. Sci. USA 87:5031). B7 is a surface glycoprotein that is expressed on activated B cells and interferon-.gamma. treated monocytes (Freeman et al., 1989, J. Immunol. 143:2714; Yockochi et al., 1982, J. Immunol. 128 823; Freedman et al., 1991, Cell. Immunol. 137:429). The binding of B7/BB1 to CD28 potentiates the level of proliferation initiated by the antigen receptor complex (Koulova et al., 1991, J. Exp. Med. 173:759; Linsley; Gimmi et al., 1991, Proc. Natl. Acad. Sci. USA 88:6575). Similarly, the inability of fixed accessory cells to induce T-cell response can be corrected by ligation of CD28 with allogeneic accessory cells or antibody (Jenkins et al., 1988, J. Immunol. 140:3324; Harding). Engagement of the TcR.zeta./CD3 complex in the absence of CD28 ligation leads to a state of anergy. The requirement for this second signal may play an important role in vivo in establishing tolerance in the T-cell periphery to antigens that were not encountered in the thymus.